The present application is based on Japanese Patent Application No. 2001-219175, which is incorporated herein by reference.
The present invention relates to a hydrodynamic bearing device in which a shaft member and a bearing member are supported relatively rotatably by dynamic pressure generated in a predetermined lubricating fluid.
In recent years, various proposals have been made concerning hydrodynamic bearing devices for rotatably supporting various rotating members such as magnetic disks, polygonal mirrors, and optical disks at high speed. For example, in a hydrodynamic bearing device adopted for a hard disk drive (HDD) shown in FIG. 29, a rotating shaft (shaft member) 2 is rotatably inserted in a fixed bearing sleeve (bearing member) 1, and a lubricating fluid F such as oil and a magnetic fluid is injected into a very small radial gap between an inner peripheral surface of the bearing sleeve 1 and an outer peripheral surface of the rotating shaft 2, thereby forming two radial bearing portions RB spaced apart in the axial direction.
In addition, as also shown in FIG. 30, a thrust plate 3 is joined to the rotating shaft 2 by such as press fit, shrinkage fit, or screwing down, and both axial end faces of the thrust plate 3, on the one hand, and the bearing sleeve 1 and a counter plate 4 attached to the bearing sleeve 1, on the other hand, are disposed in face-to-face relation with very small gaps interposed therebetween in the axial direction. Further, the lubricating fluid F is injected into the very small gaps in such a manner as to continue from the aforementioned radial bearing portion RB, thereby forming thrust hydrodynamic bearing portions SBa and SBb at two upper and lower areas on both axial sides of the thrust plate 3.
At this juncture, the bearing sleeve 1 is formed so as to shape a bag-like space in which the illustrated upper end side is in an open state while the illustrated lower end side is in a closed state. The illustrated upper ones of radial dynamic-pressure generating grooves RBV1 and RBV2 in two areas respectively formed in the aforementioned radial bearing portions RB have such an asymmetric groove shape as to generate a pumping force acting toward the illustrated lower side which is the inner side of the bag-like space of the bearing sleeve 1. It should be noted that the respective dynamic-pressure generating grooves formed in each of other hydrodynamic bearing portions are formed in symmetrical shapes, and are arranged to generate only the internal pressure in the respective hydrodynamic bearing portions.
Meanwhile, a rotating hub 5 for holding an unillustrated recording disks joined to the illustrated upper portion of the rotating shaft 2 by such as press fit or shrinkage fit, and the holding of the recording disk is effected by a damper (not shown) screwed down to the illustrated upper end portion of the rotating shaft 2.
However, when the above-described thrust plate 3 is joined to the rotating shaft 2, there are cases where deformation is caused in the thrust plate 3 due to the joining force at that time, so that there is a problem in that the bearing action in the thrust hydrodynamic bearing portions SBa and SBb fails to be satisfactory.
Particularly in a case where the thrust plate 3 is fixed by screwing, there is a high possibility of causing a noticeable problem. However, in a case where, for example, no deformation has occurred in the thrust plate 3 secured by a fixing screw 6 as shown in FIG. 31, the pressurizing force directed toward the radially inward side (pumping-in) in the thrust hydrodynamic bearing portions SBa and SBb and the pressurizing force directed toward the radially outward side (pumping-out) are satisfactorily balanced. Hence, the amount of floatation of the thrust plate 3 in the thrust hydrodynamic bearing portions SBa and SBb assumes an intended state.
However, in a case where deformation has occurred in the thrust plate 3 as shown in FIG. 32 or FIG. 33, the aforementioned pressurizing force directed toward the radially inward side (pumping-in) in the thrust hydrodynamic bearing portions SBa and SBb and the pressurizing force directed toward the radially outward side (pumping-out) assume an unbalanced state. Consequently, it becomes impossible to obtain an intended amount of floatation with respect to the thrust plate 3, and there are cases where the thrust plate 3 is brought into contact with the bearing sleeve 1 or the counter plate 4, leading to a state of rotation stop.
On the other hand, even in a case where constituent parts of the bearing member including the thrust plate 3 have been assembled with high precision, there are cases where variations have occurred in various dimensions such as the length, depth, and width of the herringbone-shaped grooves making up the dynamic-pressure generating mechanism, or in a case where the counter plate 4 attached to the bearing sleeve 1 has undergone deformation at the time of joining or the like. In those cases as well, the pumping forces in the thrust hydrodynamic bearing portions SBa and SBb assume an unbalanced state, so that there is a possibility of incurring a similar problem.
The object of the invention is to provide a hydrodynamic bearing device which is capable of satisfactorily balancing the pumping forces in the thrust hydrodynamic bearing portions irrespective of the deformation of the thrust plate and other members.
(1) To attain the above object, in the hydrodynamic bearing device of the present invention, the pair of thrust hydrodynamic bearing portions are made to communicate with each other to form a fluid circulating passage for equalizing a pressure imbalance between the pair of thrust hydrodynamic bearing portions. In accordance with the hydrodynamic bearing device having the above-described construction, even in a case where a pressure imbalance has occurred in the lubricating fluid inside the thrust hydrodynamic bearing portion due to such a cause as the deformation of the thrust plate, the lubricating fluid moves between the pair of upper and lower thrust hydrodynamic bearing portions through the fluid circulating passage so as to overcome the imbalance. Consequently, the arrangement provided is such that the amount of floatation in the thrust hydrodynamic bearing portion can be obtained stably.
(2) In the hydrodynamic bearing device, the pair of thrust hydrodynamic bearing portions are made to communicate with each other to form a fluid circulating passage for equalizing a pressure imbalance between the pair of thrust hydrodynamic bearing portions, the fluid circulating passage is disposed in a region located radially inwardly of an imaginary inner peripheral circle connecting innermost peripheral ends of the radial regions where the dynamic-pressure generating mechanism extends, and the fluid circulating passage is formed such that a flow rate per unit time of the lubricating fluid passing through the fluid circulating passage from one to another one of the pair of thrust hydrodynamic bearing portions becomes greater than the flow rate per unit time of the lubricating fluid passing over a peripheral wall surface of an imaginary circular cylinder which has a bottom surface defined by the imaginary inner peripheral circle and has a relative amount of floatation as the height thereof.
In accordance with the hydrodynamic bearing device having the above-described construction, even in a case where a pressure imbalance has occurred in the lubricating fluid inside the thrust hydrodynamic bearing portion due to such a cause as the deformation of the thrust plate, the lubricating fluid moves smoothly between the pair of upper and lower thrust hydrodynamic bearing portions through the fluid circulating passage having a sufficient flow rate so as to overcome the imbalance. Consequently, the arrangement provided is such that the amount of floatation in the thrust hydrodynamic bearing portion can be obtained very stably.
(3) Further, in addition to the arrangements as described in the above (1) and (2), a radial hydrodynamic bearing portion is provided for pressurizing the lubricating fluid in the thrust hydrodynamic bearing portion toward a radially inward side, and a bearing space extending from the radial hydrodynamic bearing portion to the thrust hydrodynamic bearing portion is formed into a bag-like space whose radial hydrodynamic bearing portion side is open to an outside and whose thrust hydrodynamic bearing portion side is closed. Therefore, the lubricating fluid flows toward the inward side of the thrust hydrodynamic bearing portion, so that the severance of the lubricating fluid in the thrust hydrodynamic bearing portion can be prevented. Thus the arrangement provided is such that each action described above is maintained satisfactorily, and the leakage of the lubricating fluid to the outside is prevented.
(4) In the hydrodynamic bearing device, the pair of thrust hydrodynamic bearing portions are made to communicate with each other to form a fluid circulating passage for equalizing a pressure imbalance between the pair of thrust hydrodynamic bearing portions, the fluid circulating passage is disposed in a region located radially inwardly of an imaginary inner peripheral circle connecting innermost peripheral ends of the radial regions where the dynamic-pressure generating mechanism extends, and a total sum of cross-sectional areas of the fluid circulating passage in a direction perpendicular to a flowing direction of the lubricating fluid is set to be not less than {fraction (3/1000)} of the area of the radial regions where the dynamic-pressure generating mechanism extends. In accordance with the hydrodynamic bearing device having the above-described construction, even in a case where a pressure imbalance has occurred in the lubricating fluid inside the thrust hydrodynamic bearing portion due to such a cause as the deformation of the thrust plate, the lubricating fluid moves smoothly between the pair of upper and lower thrust hydrodynamic bearing portions through the fluid circulating passage so as to overcome the imbalance. Consequently, the arrangement provided is such that the amount of floatation in the thrust hydrodynamic bearing portion can be obtained stably.
(5) Further, in addition to the arrangement provided in (4), the fluid circulating passage is defined by an outer peripheral surface of the shaft member and an inner peripheral wall surface of a groove portion formed by notching an innermost peripheral portion of the thrust plate in such a manner as to be open to the side of the shaft member, and the total sum of the cross-sectional areas of the fluid circulating passage in the direction perpendicular to the flowing direction of the lubricating fluid is set to be not more than {fraction (1/50)} of the area of the radial regions where the dynamic-pressure generating mechanism extends. Therefore, even if the fluid circulating passage is formed in such a manner as to notch the inner peripheral wall surface of the thrust plate, the joining strength with respect to the shaft member is maintained satisfactorily.